I'd have to go with Space Quest myself. I find the universe a lot of fun, and the games themselves quite funny. Plus they have lots of cool death scenes.
Plus they're some of the few sci-fi adventure games.
Haven't played any QFG games yet, so I can't weigh in on those.
I liked the PQ games, but not as much as the SQ ones.
KQ6 seems to be the only KQ I like... the rest seem to be giving me a general feeling of "Eh" as I play through them.
And the LSL games I've played just don't appeal to me much. High on sophomoric humor, low on wit. They seem to operate on the premise that dirty comments are inherently funny (which IMHO, they aren't).
Peace & Luv, Liz